When manufacturing products made of a material such as silicon carbide or aluminum nitride, the raw material such as silicon carbide accommodated in a graphitic mold or a mold container made of graphite is heated and compressed so as to be formed into the shape of the mold or the like. When the raw material is placed directly in the mold or the like and heated in such a molding process, both the raw material and the mold or the like are heated to a very high temperature. This leads to problems such as damage to the mold or the like, which results from the reaction between the raw material and the mold or the like, and contamination to the raw material, which results from the impurities contained in the mold or the like. In addition, when an evaporated gas or the like produced from the raw material comes into contact with the raw material during the molding process, a problem arises that air bubbles form in the manufactured product, resulting in poor product quality.
In view of such problems, a mold release sheet is conventionally interposed between the raw material and a mold for molding a substance. For example, in manufacturing quartz glass, a graphite sheet or a felt material made of graphite carbon fibers is interposed between silica powder and a graphitic mold (see, for example, Patent References 1 to 3 listed below).
Specifically, the just-mentioned Patent References 1 and 2 describes the use of a graphite felt or a graphite sheet having a bulk density of from 0.1-1.5 g/cm3 that is refined to a high purity such that the content of the impurities Na, K, Fe, and Ti is 1 ppm or less. It is described that both the graphite felt and the graphite sheet have elasticity and air permeability. Moreover, it is described that the graphite sheet does not decrease in a large amount even when it reacts with silica powder, and that the graphite sheet is suitable to obtain flatness of the glass surface because its surfaces are smooth.
Patent Reference 3 describes that a carbon felt with a bulk density of from 0.1-0.5 g/cm3 made of graphite carbon fiber is interposed at a contact portion between packed silica powder and a cylindrically constituted graphitic carbon. Patent Reference 3 also describes that the carbon felt has the function to remove evaporated silica gas and the gas produced by the reaction between the carbon felt and silica, and that the carbon felt also absorbs the thermal expansion difference between the produced glass and the cylindrically constituted graphitic carbon and therefore serves to prevent them from breaking during cooling after manufacturing the glass.    [Patent Reference 1] Japanese Unexamined Patent Publication No. 11-228166    [Patent Reference 2] Japanese Unexamined Patent Publication No. 11-278857    [Patent Reference 3] Japanese Unexamined Patent Publication No. 10-167742